I. Technical Field
The present invention relates to a centrifugal compressor used for a turbocharger or the like.
II. Description of the Related Art
In the related art, for example, a centrifugal compressor used for a turbocharger or the like of an internal combustion engine for motor vehicles is known.
FIG. 13 is a front view of a principle portion of a centrifugal compressor in the related art. FIG. 14 is a vertical cross-sectional view of a principal portion of the centrifugal compressor in the related art. A centrifugal compressor 10 in the drawing compresses fluid such as gas or air introduced from the outside of a casing 11 by rotating an impeller 13 provided with a number of blades 12 in the casing 11. The flow of fluid (air flow) formed in this manner is sent to the outside via an impeller exit (hereinafter, referred also to as “diffuser section inlet”) 14 which corresponds to the outer peripheral end of the impeller 13, a diffuser section 15 and a volute section 16. Reference numeral 17 in the drawing designates an axis of rotation of the impeller 13.
The diffuser section 15 described above is provided between the impeller exit 14 and the volute section 16, and is a channel for restoring the static pressure by decreasing the velocity of the air flow discharged from the impeller exit 14. The diffuser section 15 is provided with vanes when required. With the provision of the vanes on the diffuser section 15, as shown in FIG. 15, changing of the operating range of the centrifugal compressor is enabled. In other words, with the vanes provided on the diffuser section 15, a surge line which indicates occurrence of surging may be moved at a high-pressure ratio and the side of the low flow rate. Here, the term surging means a phenomenon such that the pressure and the flow rate are varied when the centrifugal compressor generates a sort of self-excited oscillation and discharges compressed air in specific cycles, which determines the operational limit on the side of the low flow rate.
The centrifugal compressor used for the turbocharger for motor vehicles is operated in various numbers of revolutions, a wide operating range is required. However, when the flow rate is lowered in the centrifugal compressor, the above-described surging occurs in the diffuser section 15. On the other hand, when the flow rate is increased, occlusion of fluid, so-called “chocking” occurs at the impeller or in the interior of the diffuser section, and the range of the flow rate on the side of the high flow rate is limited.
In the related art, in order to widen the operating range of the centrifugal compressor, a technology to provide a groove 25 and a circulating channel 26 on a casing 21 is known as shown in FIG. 16 (For example, refer to Japanese Unexamined Patent Application Publication No. Hei 10-176699).
A technology to widen the operating range by applying a variable mechanism such as an inlet variable guide wing or a variable diffuser to the centrifugal compressor is known (For example, refer to Japanese Unexamined Patent Application Publication No. Hei 11-173300, Japanese Unexamined Patent Application Publication No.2001-329995, Japanese Unexamined Patent Application Publication No.2001-329996, and Patent No.3038398). More specifically, the variable diffuser is able to vary the channel area by rotating or sliding a diffuser vane 28 as shown in FIG. 17A and FIG. 17B, and is able to widen the operating range of the centrifugal compressor. In particular, in the variable diffuser in FIG. 17B, the operating range is widened by varying the angle of the diffuser vanes according to the flow velocity of gas discharged from the impeller 13.